反應器冷卻水泵軸鋼材敏化與腐蝕行為研究

Study of Sensitization and Corrosion Behavior of Reactor Coolant Pump Steel

SS347為18-8沃斯田系列之不鏽鋼，主要是加入合金元素Nb與Ta以防止碳化物之析出。雖然SS347通常設計使用於高溫且具腐蝕環境中，但不適當的熱處理會減弱SS347的腐蝕抗性。故而我們使用雙程動態極化掃描方法來研究熱處理溫度對SS347敏化行為的影響。在實驗上我們發現以下兩個現象：（1）敏化溫度為550℃及850℃時，不鏽鋼347敏化的程度不隨敏化熱處理時間增加而改變；550℃時敏化因溫度低，碳擴散速率慢而不顯著，850℃時因碳化鈮穩定碳的效應，使敏化不隨時間而惡化。（2）在敏化溫度為650℃及750℃時、隨著敏化時間的增加，敏化程度也隨時間增加而遞增。且成線性關係，敏化溫度650℃時敏化程度增加最快。敏化程度是碳化鉻析出與鉻原子擴散消弭鉻乏區之間動態平衡的結果。

SS347 is one of the 18-8 series austenitic stainless steels. The major improvement for this kind of stainless steel is to add alloying elements of Nb and Ta. The purpose of adding these elements is to prevent the precipitation of carbide and the consequent sensitization of the steel. Although SS347 is usually designed for utilizing in the high temperature and corrosive environment, improper heat treatment would decrease the corrosion resistance of SS347. We use the doubleloop electrochemical potentiokinetic reactivation (DL-EPR) to study the effect of heat treatment temperature on the sensitization behavior of SS347. Two phenomena were observed as follows: 1. Degree of sensitization of SS347 undergoing 550℃ and 850℃ heat treatment wouldn’t change as the sensitization time increases. Low carbon diffusion rate limits the formation of chromium carbides at 550℃. Niobium catches more carbon atoms to prevent the formation of chromium carbides at 850℃. 2. Degree of sensitization of SS347 undergoing 650℃ and 750℃ heat treatment would increase linearly as the sensitization time increases. D.O.S. is the result of a balance between the rate of the precipitation of chromium carbide and the rate at which the chromium-depleted zones are eliminated by diffusion.